In a DL MU-MIMO system the eNodeB transmits data to different UEs on the same time-frequency resources from multiple transmit antennas. To minimise interference between UEs, the eNodeB creates transmission beams through precoding. At the receiving site, a UE uses post-coding (decoding) to take (decode) its data.
In the DL MU-MIMO system, also the eNodeB can transmit different data to one UE on the same time-frequency resources from multiple transmit antennas. The number of the data for one UE is defined as rank.
In communication systems, feedback about the status of the downlink channel between a transmitter and a receiver is employed in order to optimise transmission of data over the downlink channel. The receiver determines the downlink channel status information (CSI) from received pilot signals, and then communicates this CSI to the transmitter, or UE determines the downlink CSI from received pilot signals, and then communicates this CSI to eNodeB, for use in altering the rank, precoding matrices, coding rate and modulation scheme for downlink transmission used in subsequent data transmissions. In 3GPP TS36.213 (V10.5.0), the channel status information includes rank indication (RI), precoding matrix indicator (PMI), and channel quality indicator (CQI). PMI and RI are used for DL MIMO systems.